Nothing prevents a decorator defined using `@decorator` from
changing the signature of the decorated function. For example,
this example changes the return type to `str`:

```
from decorator import decorator

@decorator
def stringify(f, *args, **kwargs) -> str:
    return str(f(*args, **kwargs))
```

The old signature caused false positives in internal Dropbox code.

I couldn't come up with a signature that would produce better types
with mypy while not generating false positives.